1. Technical Field of the Invention
The present invention relates to a fluid dynamic bearing motor, and more particularly to a fluid dynamic bearing having a conical shape to enable the motor to be smaller in thickness and lower in cost.
2. Description of the Related Art
There has been a trend toward the fluid dynamic bearing motor as the power source for rotary memory devices, cooling fans, and the like, because of its quietness in operation and the necessity to reduce nonrepeatable runout (NRRO) of rotating parts. Portable applications of such electronic devices have been widespread, increasing the demands for further reduction in their thickness and required current. However, there are limitations on further reduction in thickness of the fluid dynamic bearings, because they need to have a certain span between the bearings for supporting the shaft in order to inhibit NRRO. Also, in order to maintain a constant clearance between the bearings, they must be machined with extreme precision in the order of submicrons, whereby it is difficult to produce them at low cost.
In order to make fluid dynamic bearings thinner, a novel structure is necessary which does not require two bearings for supporting the shaft at axially spaced positions. The bearings should have as little sliding area as possible so as to achieve a reduction in the required current. Further, cost reduction will be achieved through the development of a structure wherein the bearing clearance is maintained with necessary accuracy even with the components machined with a lower degree of precision.
Single cone fluid dynamic bearings, which can support loads of both radial and thrust directions, have attracted attention as having potentialities in many respects. However, while some single cone structures that help decrease the thickness of the bearing have been proposed, for example, in Japanese Laid-open Utility Model Publication No. Hei. 06-004731, these are for air dynamic bearings and anyway have not been very successful. The main reason is that the single cone bearing is structurally incapable of sufficiently inhibiting NRRO during rotation. Japanese Laid-open Patent Publications No. 2000-004557 and No. 2000-205248 propose combined use of a conical bearing and a cylindrical bearing to improve the overall performance. However, the cylindrical bearing requires high-degree machining precision for maintaining a constant bearing clearance, canceling out the advantages of the conical bearing. U.S. Pat. No. 5,854,524 discloses a single semi-spherical air dynamic bearing having a similar structure as that of the single cone bearing, but in this case also, the radius of two spherical surfaces must be strictly controlled to secure a sufficient radial load capacity, because of which cost reduction is hardly achievable.
Thus the problems yet to be resolved in single cone fluid dynamic bearing motors are how to improve the stability in its rotating attitude, and how to realize a structure which prevents leakage of the lubricant and yet is easy to assemble.
An object of the present invention is to resolve these problems and to provide a single cone fluid dynamic bearing motor which can be reduced in thickness and required current, and is simple and can be produced at lower cost.
A fluid dynamic bearing motor according to the invention includes a shaft having a diminishing conical taper surface, a sleeve having a conical concavity opposite the shaft, lubricant filled in a clearance between the shaft and the sleeve, and means for generating magnetic attraction between one end of the shaft and a cone apex of the sleeve, and an annular wall arranged around the shaft to face an outer circumferential wall of the sleeve, a clearance between the annular wall and the outer circumferential wall of the sleeve being increased in width toward an open end to form a taper seal of the lubricant. In this construction, a plurality of grooves are formed on a conical taper surface of one of the shaft and the sleeve, and the grooves are provided for creating load capacity when the motor rotates, whereby rotating parts of the motor are supported by axial components of the load capacity balanced with the magnetic attraction. The boundary of the lubricant is positioned around the sleeve, so as to enable a reliable seal to be formed even in high-speed operation.
A ring-shaped member is fixed to one end of the annular wall which is arranged around the shaft, and an annular recess is provided in the outer circumferential wall of the sleeve, the inner periphery of the ring-shaped member being positioned within the annular recess, so as to restrict an axial movable distance of the rotating parts. This structure serves as a stopper for the rotating parts in the case where the motor is subjected to a large shock.
The means for generating magnetic attraction includes a permanent magnet and a magnetic material, respectively provided inside the shaft and in the apex of the sleeve opposite to the shaft, or vice versa. Magnetic attraction developed at the end of the shaft acts on the shaft to adjust its position in cooperation with the load capacity created by the grooves, thereby ensuring the stable attitude of rotating parts.
Moreover, the following structures for a permanent magnet to protrude from one end of the shaft are proposed. The shaft includes a permanent magnet held inside. The permanent magnet is assembled with the shaft such that it is initially held movably but firmly enough to overcome the magnetic attraction as being substantially protruded from one end of the shaft, and is pressed into the shaft by a pressure larger than the magnetic attraction applied from both ends of the shaft and the sleeve to a predetermined position, where the cone apex of the sleeve or a plate spring interposed between the apex of the sleeve and the permanent magnet is resiliently deformed, whereby when the motor is stationary the permanent magnet and the apex of the sleeve or the plate spring make contact with each other, while they are brought out of contact when the motor is rotating, by a distance equal to or shorter than an axial flying height determined on conical surfaces of the shaft and sleeve. Thereby, the start-up failure caused by the conical surface of the shaft being fitted in the sleeve when the motor is not in operation can be avoided, improving the reliability of the motor.
Alternatively, the grooves may be formed on both opposite taper surfaces of the shaft and the sleeve at the almost same axial positions. In this constitution, the grooves have different angular length from each other in the circumferential direction. Thereby, each delay, from the time point when the bearing clearance becomes small until the time point when the pressure in the lubricant in the clearance becomes local maximum by the corresponding groove, is varied in proportion to the corresponding angular length of each of the grooves. Thereby, an improved constitution which can avoid half whirls and other unstable movements of the motor can be provided.
Alternatively, conductive magnetic powder may be mixed in the lubricant so as to electrically connect the rotating parts to ground because of the powder gathered in the magnetic field between the end of the shaft and the apex of the sleeve.
According to the fluid dynamic bearing motor of the present invention, the load capacity created by the rotation of the motor acts vertically with respect to the conical surfaces, causing the shaft and the sleeve to rotate in non-contact relationship at a position where the axial components of the load capacity and the magnetic attraction are in equilibrium. The radial components of the load capacity counterbalance each other at respective circumferential points, thereby contributing to the centering of the rotating parts. The load capacity itself acts vertically on the tapered surface of the cone, and therefore it serves to adjust the attitude of rotating parts when they tilt with respect to the fulcrum conforming to the cone apex.
Magnetic attraction developed at the end of the shaft acts on the shaft to adjust its position in cooperation with the load capacity created by the grooves, thereby ensuring the stable attitude of rotating parts.
The main reason why the prior art single cone bearing has failed to maintain the attitude of rotating parts is that the bearing was provided only with a load equal to the weight of its own, or even less than that by using a magnetic bearing in order to avoid friction during the initial and final periods of operation as disclosed in Japanese Laid-open Utility Model Publication No. Hei. 06-004731. As has been explained above, a good balance is achieved between two forces of the axial component of load capacity of the bearing versus the load. Therefore, a small load can only create a small load capacity, which is insufficient to create forces for maintaining stable attitude of rotating parts. In the fluid dynamic bearing of the present invention, a large load is applied on the bearing by the magnetic attraction acted between the shaft and the sleeve. Therefore, the load capacity of the bearing, which counterbalances the load, can be set to a desired large value, whereby the stability of the attitude of rotating parts is improved. The magnetic attraction may be varied case by case depending on permissible level of NRRO, the size of the motor, and various other conditions.